This invention relates, in general, to a method for plasma etching organic materials and, more specifically, to a method for anisotropically etching organic materials in a hydrogen plasma.
The increasing complexity of semiconductor devices has required technological changes in the areas of materials, lithography, and processing. One such required change is the need for an anisotropic etch process for organic films. Two organic films which are now commonly used in semiconductor processing and which, in some applications, require anisotropic etching are, for example, polyimides and photoresist.
Polyimide is an organic material of increasing importance in semiconductor device processing because it possesses desirable dielectric and passivation properties. Current wet chemical as well as high pressure plasma etch processes for polyimide, however, produce lateral etching which, at best, is proportional to the vertical etch depth. The trend in semiconductor devices is towards smaller and smaller device geometries with closely spaced components and fine pattern sizes. As the device geometries continue to shrink, so also do the required openings which must be cut through the polyimide layers. The necessity for small closely spaced openings through polyimide layers requires an anisotropic polyimide etch process.
Two new process technologies which are becoming important are reactive ion etching and ion milling. Both of these processes require thick organic masking layers to pattern an etchable material because resist attack is so severe with these etch processes that it may be the limiting factor in determining the success or failure of the etch process. In addition, a thick resist is required in the process both for good step coverage and to minimize standing wave patterns caused by interference from reflected light. The need for thick resist layers is seemingly incompatible with high resolution and close dimensional control from a lithographic standpoint since the latter are usually best obtained in thin resist layers, typically less than 400 nanometers. One way to pattern the thick organic masking layer and to simultaneously maintain high resolution and dimensional control is to use a trilevel process. In that process a thick organic film of photoresist or polyimide is first applied to the substrate. The thick organic film will provide the ultimate masking layer and additionally serves to planarize the underlying substrate. Over the thick organic layer is applied an inorganic intermediate masking layer (a "hard" mask) and then a thin top layer of x-ray or e-beam photoresist. In using the trilevel process the top layer of photoresist is patterned in a desired fine geometry pattern. This in turn is used as an etch mask to pattern the hard mask. The hard mask is then used as an etch mask to pattern the thick organic layer. Etching the thick organic layer requires an anisotropic etch to replicate the pattern provided by the hard mask.
There are a number of ways to etch organic materials. It is well-known, for example, that organic films can be etched in an oxygen plasma; oxygen plasma etching or "ashing" of photoresist is common in the microelectronic industry. Ashing of a masked organic layer in an oxygen plasma, however, is an isotropic process resulting in severe undercutting of the mask material. Likewise, liquid etchants etch organic layers isotropically.
Reactive ion milling is known to be an anisotropic process with operating pressures on the order of 1.3.times.10.sup.-2 Pa. The low pressure results in a longer mean free path and better ion directionality which achieve the anisotropy. Under such conditions organic layers can be etched anisotropically with little undercutting of the mask layer. In many applications of ion milling, however, severe attack of the mask material makes good line width control difficult. In addition, reactive ion milling is characterized by very low throughput and high system cost because of the low vacuums required.
In view of the need for a process for anisotropically etching organic films and further in view of the difficulties associated with present anisotropic etching methods, a need existed for an improved anisotropic etch process.
It is therefore an object of this invention to provide an improved method for anisotropically etching organic films.
It is another object of this invention to provide an improved method for etching organic films without high vacuum equipment.
It is a further object of this invention to provide an improved method for etching openings of predetermined sidewall contour through a layer of organic material.
It is yet another object of this invention to provide an improved method to etch openings in an organic layer without undercutting the etch mask.